If I look a fresnel node trough an emissive shader what reflectance curve do I see?
I'm answering my own question because I start understanding a bit more about the topic of polarized light.
I'm niether a scientist nor a mathematician so I will simplify things but don't hesitate to correct me or give more information in the comment so I can update/complete the answer.  

What I know :

After reading more about Polarization :  


A ray light is a wave moving multiple directions, 2 plans x and y (non polarized)  
A non polarized light is P polarized after bounced on a dielectric material (water/glass/plastic...).  
Bounces on metals are non polarized.  
We haven't have spectral rendering in Cycles, so the light isn't acting as a wave, no polarization of rgb light.


What I assume :


Light in Cycles must be think like non polarized
Defaut fresnel considerate that most light is polarized because it seems to  apply a P polarized curve on fresnel
Direct ray need non polarized fresnel curve, and after one bounce on dielectric surface, it need P polarized fresnel curve.


Edit :

The second assumption isn't right because I was assuming that the drop down of the curve was a sign of polarization, in fact it's a sign that default fresnel node isn't suitable for metallic shaders (dielectrics are ok).
The third assumption isn't right either, first because if a ray hasn't bounced on something there is no fresnel curve, second because polarized light is way more complicated than that, and we haven't got any way to know which ray is polarized or not. So the best choice is to use unpolarized curve.